Generalized connection network

ABSTRACT

A generalized connection network includes a first and second sub-network interconnected by a branching circuit having N inputs and N outputs. The branching circuit has N, two-state branching elements interconnected so as to be able to replicate a signal coupled to any one of the inputs to each of up to N outputs. The cyclic connection scheme provided by an additional two-state switch permits a net reduction in the total number of two-state switches necessary to form a generalized connection network.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to generalised connection networks of particular,but not exclusive, application to optical networks.

2. Related Art

Optical space switches can provide broadband switched connections in PBXand local network environments. Generalised connection networks (GCNs)offer the additional facility of broadcasting, for example, from any oneof N inputs to any number up to N of outputs enabling any customer in alocal network, for example to become a broadcast service provider to anycombination of the other customers. The smallest GCNs published to dateoperate by separating the broadcast function into two parts; an initialreplication network (generaliser) to generate the required number ofcopies, followed by a one-to-one switching network (connector) toconnect the copies to the appropriate outputs. This segregated approachrequires more crosspoint switches than the Nlog₂ N theoretically neededto provide all of the N^(N) possible permutations. GCNs using make/breakcontacts require 5.8Nlog₂ N, and when using optical changeover switchesthey require in the order of 2Nlog₂ N-N+2.

FIG. 2 of Sakata et al "Synthesis of Multiconnected Switching Networks,Electronics and Communications in Japan, vol, 58-A, no. 1, 51-58(1975)", shows a GCN in which a branching network is positioned betweena left-hand and a right-hand sub-network made up from two input, twooutput, 2-state branching elements whose task is either to allow bothinputs through unaltered, or to copy one input, the upper input, to bothoutputs. In optics, where 2×2 changeover switches naturally perform acrossover function, their use as branching elements requiresdisconnection of the lower input line when copying, and this is easilyprovided by turning off the light source not seeking connection throughthe network or by adding a net of N additional on/off switches. A signalat a given input can be copied to a subset of outputs of the branchingnetwork as desired by passing sequentially through the switches in adownward direction. The right hand permutation network theninterconnects the output of the branching network to the desired outputsof the GCN.

SUMMERY OF THE INVENTION

According to the present invention a generalised connection networkcomprises a first and a second interconnection sub-networkinterconnected by a branching network having N inputs and N outputscharacterised in that the branching network has N, two-state branchingelements interconnected so as to be able to replicate a signal coupledto any one of the N inputs to each of up to N outputs.

This is achieved by connecting N branching elements to provide a cyclicconnection scheme (cylindrical symmetry) around the branching networkwhich then permits any degree of branching, i.e. replication, of asignal on any one of the N inputs of the branching circuit because thebranching elements form a complete cycle. This is in contrast to thepreviously mentioned Sakata et al arrangement which uses only N-1branching elements which allows the i^(th) input (where i=1 to N) to bereplicated only (N-i) times.

The applicant has realised that the inclusion of the additional twostate branching element in the branching network allows for a reductionby more than one two-state switch in the total number of two-stateswitches in the first and second Sub-networks thereby achieving a netgain in terms of two state switching elements needed to form ageneralised connection network. In particular an 8×8 GCN according tothe present invention requires a second sub-network having only 12 twostate switches in contrast to the 17 needed for the Sakata GCN.

It will be understood that each two-state branching element may be madeup one or more simultaneously operated switch elements depending, forexample, on the technology and switch topology chosen.

Each branching element may have two branching element inputs and twobranching element outputs and be switchable between a first state inwhich each branching element output is coupled to a respective branchingelement input and a second state in which both branching element outputsare coupled to the same branching element input. These may beimplemented as optical waveguide switches, for example.

Greater flexibility can be achieved in the branching network leading tofurther two-state switch reductions if each of the N inputs is coupledto a respective one of the N outputs by a guide and includes N branchingelement switchable between a first state in which an inputs coupled toeither one of two guides replicated to the other of the two guides and asecond state in which no replication occurs. That is, the branchingnetwork is configured to replicate signals in both directions, i.e. upas well as down.

In electrical technology, this may be easily achieved with make/breakcontacts for example. In optical technology this two-directionalbranching can be achieved by providing guides which comprise parallelintegrated optics waveguides and the branching elements compriseelectrodes which control coupling of an optical signal from one guide toanother.

It has been shown that it is possible to have GCMs in which the twosubnetworks have at most Nlog₂ N, two-state switches.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, by reference to the drawings in which

FIG. 1 is a schematic diagram of a 4×4 GCN according to the presentinvention employing a one-way branching network;

FIGS. 2(a) and 2(b) are schematic diagrams of the two states of theswitching elements used in the FIG. 1 embodiment;

FIGS. 3(a) and 3(b) are schematic diagrams of the two states of thebranching elements used in the FIG. 1 embodiment.

FIG. 4 is a schematic diagram of an 8×8 GCN according to the presentinvention employing a one-way branching network;

FIG. 5 is a schematic diagram of a 4×4 GCN according to the presentinvention employing a two-way branching network; and

FIG. 6 is schematic diagram of a further embodiment of 4×4 GCN employinga two-way branching network.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to FIG. 1 a 4×4 GCN has a first and a second sub-network. 2and 4, interconnected by a four-input, four-output branching network 6having four inputs I₁ -I₄ and four outputs O₁ -O₄.

The sub-networks 2 and 4 are formed from 2×2 changeover switches 8having the two states shown schematically in FIGS. 2(a) and 2(b)respectively, that is they allow signals on one input to be selectivelycoupled to either one of the outputs, the other input being coupled tothe other output.

The branching network 6 is formed from four branching elements 10 eachhaving two branching element inputs and two branching element outputs.The elements 10 have the two states shown schematically in FIGS. 3(a)and 3(b). That is, each branching element 10 either connects each outputto a respective input or both outputs to the one of the inputs.

The present invention does not rely on the actual construction of thebranching and switch elements to implement the two-state function. Anysuitable implementation may be employed.

The branching network 6 is of the same type as the branching network ofSakata et al referenced earlier but includes the additional branchingelement A which provides replication of the signal from the bottom mostoutput to the input of the top most branching element. This permitsreplication of the signal entering at any of the inputs I₁ to I₄ of thebranching network 6 up to the maximum four copies at the outputs O₁ toO₄.

The 4×4 GCN of FIG. 1 is one of two networks that have been found by theapplicant to provide all of the 256 possible connection permutations ofa 4×4 broadcast network (N^(N) =4⁴) within the 2,048 states of its 11switches. The required switch states were obtained by manual inspectionfor each permutation. The network uses just 3 more switches than thetheoretical minimum of 4log₂ 4=8.

Referring now to FIG. 4, an 8×8 GCN according to the present inventioncomprises a first and a second sub-network 14 and 16 and a branchingnetwork 18 composed of switching elements 8 and branching elements 10,respectively, as used in the FIG. 14×4 GCN. Again, full cyclicreplication of the inputs to the branching network 18 to its outputs isobtainable by means of the inclusion of the branching element A. In thisembodiment the first and second sub-networks 14 and 16 togetherconstitute a single permutation network (connector) having one morestage of switches than the minimum permutation networks of Goldstein andLeibholz, "On the synthesis of multiconnected switching networks", IEEETrans 1967 2-29 (11) pp 1029-1032. This is one of the four networks thathave been found capable of 8×8 broadcast switching with left andright-hand sub-networks each having log₂ N=3, fully filled stages. Allfour networks employ the same number of two-state switches, but haveslightly different link patterns between the first sub-network and thebranching network.

In these 8×8 cases the number of connection permutations (8⁸=16,777,216) made manual inspection impossible to obtain the switchstates. Proof that all permutations could be satisfied was obtainedinstead by exhaustive computer search through all 4,294,967,296 statesof the 32 switches. The 8×8 network of the present invention uses 8 moreswitches than the theoretical minimum of 24.

Referring now to FIG. 6 an experimental 4×4 GCN has discrete opticalcomponents in a two-way branching network 20, to demonstrate theoperation of the structure without requiring a special integrated-opticdevice to be fabricated. The branching network 20 consists of 3×3single-mode fused-fibre couplers 22 which split the optical power fromeach input to the branching network 20, and 2×2 changeover switches 24which act as on/off switches to control the upward and downwarddistribution of signals between the lines. (This discrete approach doeshowever increase the number of switches but they still are two-statedevices considered as a whole). The manually derived switch states werechecked on the network, and all 256 permutations obtained.

A further optical GCN network is shown in FIG. 5, which uses parallelintegrated-optics waveguides throughout, with electrodes 26 for couplinglight into adjacent waveguides in both directions, i.e. upwards anddownwards.

The two-state switch B is formed as an electrode coupling to anadditional waveguide 38. The cyclic nature is obtained in this case, bylinking the waveguide 38 by optical fibre links 36 and 42 round to thewaveguide 44 to which they are passively coupled.

Only 9, two-state switches are now required, which is just one more thanthe theoretical minimum. The first and second sub-networks 28 and 30 ofthe embodiment of FIG. 5, excluding the branching network, togetherconstitute precisely one 4×4 minimal permutation network (connector),with no additional switches required at all.

Networks according to the present invention are equally applicable totechnologies other than optical technologies, especially electronicswhere 2×2 changeover switches and branching elements and switches can bereadily implemented using logic or analogue gates.

I claim:
 1. A generalised connection network comprising:a first and asecond interconnection sub-network interconnected by a branching networkhaving N inputs and N outputs, N being an integer, the branching networkhaving N two-state branching elements interconnected so as to be ablesimultaneously to replicate a signal coupled to any one of the N inputsto each of up to N outputs.
 2. A network as in claim 1 in which eachbranching element has two branching element inputs and two branchingelement outputs and is switchable between a first state in which eachbranching element output is coupled to a respective branching elementinput and a second state in which both branching element outputs arecoupled to the same branching element input.
 3. A network as in claim 2in which in the second state both the branching element inputs arecoupled to both branching element outputs.
 4. A network as in claim 1 inwhich each of the N inputs is coupled to a respective one of the Noutputs by a guide and including N branching elements switchable betweena first state in which an input coupled to either one of two guides isreplicated to the other of the two guides and a second state in which noreplication occurs.
 5. A network as in any preceding claim in which thebranching elements include optical waveguide switches.
 6. A network asin claim 4 in which the guides comprise parallel integrated opticswaveguides and the branching elements comprise electrodes which controlcoupling of an optical signal from one guide to another.
 7. A network asin any preceding claim in which the total number of two state switchesin the sub-networks is no more than Nlog₂ N.